balun

A balun, short for balanced-unbalanced, is an electrical device that converts between balanced and unbalanced signals and impedance, while preserving signal integrity and suppressing unwanted currents on the feed line. The term balun is a portmanteau of "balanced" and "unbalanced." A balanced line carries equal and opposite currents on two conductors, with the line impedance independent of the shield, whereas an unbalanced line uses a single conductor relative to a common return.

Baluns come in several forms. Transformer-based baluns use a transformer to couple energy between balanced and unbalanced sides and may include additional windings to provide impedance transformation, such as 4:1 or 9:1 ratios. Current baluns, also called choke baluns, rely on a common-mode choke to suppress currents on the outer conductor without significantly altering the differential signal. Coaxial baluns combine coaxial line and a transformer or winding on a ferrite core. In practice, 1:1 baluns block RF currents on the feed line, while 4:1 or 9:1 baluns transform impedances to suit the receiver or transmitter.

Typical uses include feeding balanced dipole antennas from unbalanced coax, where the balun helps maintain antenna symmetry and prevent the feed line from radiating or picking up interference. They are also used to connect balanced line systems to unbalanced equipment, and in some TV and communications systems to transition two-wire lines to coax.

Construction and performance depend on bandwidth, core material, and losses. Commonly used ferrite cores and winding configurations allow operation across a range of frequencies, with attention to RF currents, impedance, and the desired balance transformation.